This invention relates to continuously operating mixers and extruders for flowable media mainly, but not exclusively, of the visco-elastic kind, having an inlet, an outlet and between these at least a mixing/plasticising section of the Transfermix type.
GB-A-842 692 shows a Transfermix section, relating particularly to mixers and extruders comprising a driven rotor and a stator having respectively an internal and an external helical groove, the grooves being coaxial but of opposite hand and the helical groove in the one component, e.g. the rotor, varying from a full flow cross-section to zero cross-section over the axial length of one Transfermix section while the helical groove in the other component, e.g. the barrel, varies from zero cross-section to full cross-section over substantially the same axial length, and vice versa in a possibly following Transfermix return section. In operation, the material being transported initially in the one component at the entry to the Transfermix section is transferred layer-by-layer from this as the giver-component to the other as the taker-component until by the end of a Transfermix section it will have been transferred, mixed and worked layer by layer in an orderly sequence.
A Transfermix section provides the property that throttling of the flow material changes the intensity with which the throughput is being influenced, more particularly the mechanical work input, in any case uniformly but to a degree of uniformity which depends upon the geometrical design of the Transfermix section.
A first generation of Transfermix is described in GB-A- 842 692, while GB-A- 1 585 531 and 1 585 532 show a second generation with multiple grooves, and EP-B-0 574 172 shows a third generation with longitudinal mixing.
In extruders or mixers having the function of shaping an extrudate through a die at the end of the outlet-section, the amount that the medium is throttled is predetermined by the cross-sectional shape, form and length of its flow-channel. However, the concern of this invention is with additional throttles for influencing the amount of work input into the medium generally and compensating for different throttle-effects inherent in different dies that may be used, in order to obtain a required quality in the first place and the same quality of the extrudate out of different dies, in the second place. Throttling devices could also be formed by screens situated at the outlet end of the screw, if required for the removal of particulate impurities from the extrudate, but their throttling effects are a given quantity similar to the resistance of dies. However, backing plates for such screens can form a step-wise adjustable throttle, particularly if used in some form of screen changer. The backing plates have different sizes of holes as originally needed to support different screen-packs composed of wire-mesh situated past the end of the rotor screw.
A continuously adjustable throttle at that position in the extruder is described particularly in GB 1 585 532. This throttle is in the form of conically-ended pins which can be moved radially into and out of the circular flow-cross-section, or even of a single pin of a diameter almost equal to that of the circular flow-cross-section and with a rounded end which can be moved across the flow to fit the opposite wall of the channel. Practical work with such throttles established that with Transfermix sections of suitable intensity, as available for the second generation of Transfermix, very difficult-to-plastify rubber compounds--at that time natural rubber compounds with high loadings of fine carbon-blacks--could be satisfactorily plastified with only less than 5% of the cross-sectional area being left available for flow. The throughputs were still acceptably high, as explained below, although this probably occurs only with a Transfermix section of suitable geometry.
If an easy-to-plastify compound with no throttling reached an output of satisfactory quality with an output of X Kg/hr, then the difficult-to-plastify compound would, at the same screw speed, provide an output of 2, 3 or 4 times this quantity, albeit of insufficiently plastified material with cold lumps in it and a knobbly surface and probably running unstabily. Throttling would then provide improvement in plastification, frequently enough down to a throughput of a similar magnitude as X Kg/hr, or perhaps down to around 50% of X. This would still be very satisfactory, when extruders with other plastifying sections would not permit sufficient plastification at any throughput.
Such throttles have the following drawbacks:
1. They produce a pressure peak at the end of the screw when put into action, so that, in addition of the desired action of slowing down the flow in the plasticising section, they produce a pressure back-flow in the transport screw between the Transfermix section and the end of the screw. This produces an unnecessary build up of heat, which may be a limiting factor for certain compounds. PA1 2. When the throttle pins are withdrawn completely, they leave openings in the cylindrical wall of the casing in which compound cannot be moved and is in danger of curing-up and then in later running contaminating the flow. PA1 3. Whereas a Transfermix section is self-cleaning up to the end of the screw, this type of throttle holds quite an amount of rubber when operation ceases which cannot be got out even with an extrusion head which is openable for cleaning. This is a disadvantage on changing compounds.
EP-A-0 509 779 (Meyer) shows a throttle with pins having frusto-conical ends situated in the barrel immediately after the Transfermix section and operating into a circumferential cut in the transport screw, thereby doing away with any unwanted heat build-up up to the end of the transport screw. However, it has the second disadvantage mentioned above. Even where the frusto-conical pins are without any internal pins permanently in place, as described, the necessary gap in the screw additionally reduces its transport and pressure-buildup action.
EP-A-0 490 362 (Capelle) shows radial pins, radially adjustable, in the deepest grooves of the barrel at the transition section between a first and a second Transfermix zone. While avoiding the three disadvantages quoted above, the clearance gaps between the pins and sides of the helical grooves in the barrel, see FIGS. 5 and 6, must be of a considerable magnitude in order to prevent trapping and curing up compound in the grooves themselves when the throttle pins are moved in. This feature prevents a closure to anywhere near 95%--more likely remaining well under 90% or even less, calculated as a percentage of the total flow area of the stator grooves, especially as the necessary clearance between internal thread-lands of the barrel and the opposite ungrooved surface of the screw has to be added. In this way the plastification of a number of compounds at the difficult end of the range is excluded.
EP-A- 0 345 687 (H. D. Wagner and H. Holzer) shows an adjustable barrier at the end of a plasticizing section of a vacuum extruder, comprising two relatively adjustable rings, one fixed and the other one rotatable, which when opened have axially co-extending gaps and by means of rotation can close these to a required degree.
This arrangement has been used at the position between a first and second Transfermix section, where the first Transfermix section has a geometry of the third generation, including longitudinal mixing, and where this first Transfermix section, in which the plastification has mainly to be completed, is long and the return section is as short as is needed not to restrict the flow back from the barrel into the screw. This length is generally about the same as the maximum depth of the helical thread in the barrel. With a design of radial "teeth" and gaps being of equal width, this permits throttling of 90-95% of the annular flow, taking into account also the necessary radial clearance between screw and barrel at that position. However, when open it can be free to little more than 50% of the flow-cross-section. With teeth and gaps of lesser width, that much closure is no longer possible while complete opening is still impossible. This construction, while constructionally simple, has the disadvantage of not providing self-cleaning on the feed of rubber being stopped and the screw being run empty for cleaning. Self-cleaning is generally very useful and a major need in uses where compounds need to be changed with any frequently and particularly where compounds with metal-adhesive properties are concerned.
EP-A-0 587 574 (Meyer) shows the third generation Transfermix geometry applied to rubber injection moulding machines of the type, where the screw, after effecting plastification into a cylindrical continuation of the barrel as the reservoir, is then pushed forward to provide the piston-action for injecting the compound into the mould. Both the rotation of the screw and its axial motion are frequently effected by oil hydraulic means. As controlled axial positioning of the screw between fully back, for plasticizing, and fully forward, for complete injection, can easily be made a feature of this application and frequently is installed anyway, the use is described of such positioning for continuously adjustable throttling between the end-edge of the second transfer zone in the barrel and the starting-edge of this zone in the screw. This provides for a maximum closure equal to the clearance between the rotor and the barrel, and for a complete opening when the screw is fully withdrawn.
The functional disadvantage here is, however, that axially displacing the screw makes the beginning and the end of the first transfer zone geometries in screw and barrel respectively no longer coincident, that is, the Transfermix action in this, the determining mixing-plasticising section, is interfered with. Another disadvantage arises for an extruder which is normally driven by an electric motor through a reduction gear. Here a device for axially moving and precisely positioning the screw relative to the barrel is quite expensive. The same holds, if the relative axial movement were to come from shifting the barrel-assembly including the extrusion-head relative to the base of the reduction gear casing, besides upsetting the positioning of the die relative to the follow-on machinery.